What is this black bar in the Dopesheet and is it the reason I can't play my animation?
You set Start and End Frame of your Animation to the same Frame. This means it plays your Animation, but because it is only one frame long, it loops over the same frame. The black Bar is your Animation Range, 1 Frame long. Therefore it is shrunk to a line.


Current Playhead Positiion
Start of the Animation (set it to 0 in this case)
End of the Animation (set it to 450)